2003 ST 4 Stroke 9.9/15HP Johnson outboards Service Manual, Page 151Get this manual
POWER UNIT WATER COOLING SYSTEM
WATER COOLING SYSTEM
Operation
The water cooling system consists of the water pump (in the lower unit), water tube (between lower unit and power unit) and thermostat (in the cylinder)This system cools both the power unit and the exhaust and is shown in schematic form belowIf overheating occurs, the components of the cooling system must be inspected for blockage, corrosion build-up or damageComponent inspection Refer to page Water pumplmpeller 8-10 Water Tube .8-10 Thermostat 6-39 Cylinder Head 6-15 Cylinder Block 6-28